Preview Follow-up

 * Removed util file that got imported accidentally
 * Default preview expire settings are 15mins/30mins
 * Added validation check if expires is int back in
 * Added doc for min/max calculation
 * Added check + test for negative signature duration
 * Renamed Telemetry strings
 * Removed unneeded import in oae-rest/api.authentication
 * Moved downscaling logic to ImageUtil and use gm module